I'm just a low life Plat adc main with 560k points on Xayah. Xayah is no doubt had it really rough this year, even though she got multiple buff this season hoping it will becomes stronger it never really happened. So i took in condideration all of her flaws and and tried to improve on her gameplay.

She got only 525 range. That's the second least range an adc could have currently, but all the other adc who has this many or fewer, has a dash or range booster to help them out. The only other adc who also lacks these tools is Sivir, but unlike Xayah, Sivir's playstyle evolved, and it no longer relies on her auto attacks, but on her very long range Q.
Galeforce tries to make it better but a 90s dash won't make you a lucian

CRIT DAMAGE GOT NERFED
Since the beginning of season 11 it only gives you 175% bonus damage, and Infinity edge only boost it to 210%, This in directly shot Xayah in the kneecap big time for her W
Here's some math to show you what i mean, let's say you have 250ad and you crit while your W is up:
Pre S11- 250*2*1.2= 600 damage
Post S11- 250*1.75*1.2= 525 damage
Taking off that much damage form your auto attack hurt the champ a lot, it was already really risky to go in and attack someone, but now it's even less worth it then ever before, not to mention they did not buff base damage or her W to make up for it, instead, they buffed all of her other abilities.

And Her E does scale from crit it's true.. but not fast enough and as much as it should be with normal build.

First, let's take a loog at all the other 3 viable options and why are they bad for xayah:

Kraken slayer:
You need to auto attack mainly in order to get the most out of the item which we already discussed that it's really bad for Xayah, it does not give any deffensive stats, the mythic bonus gives AS and Xayah does not require that many, it becomes useless on her.

Galeforce: The main item people usually build on her, it gives a dash with bonus magic damage that scales with hp and the enemies missing health but only on a 90s cooldown, it does not give enough offensive stats for a good 1 item powerspike, it gives 3% bonus movement speed as mythic bonus, 12% max, good, but it will not help you if you are doing paper cuts even to the enemy Sona

Shieldbow: It's just bad in so many ways. Even gives less damage as Galeforce, it only gives lifesteal, not physicalwamp, so onceagain, you need to constantly auto attack in order to use it on it's full potential, the shield is good it doesn't really matter if you can stay alive a bit longer if you don't have any damage to deal with it, mythic bonus gives better offensive and deffensive stats than Galeforce or Kraken, but it does not make up for all the flaws that i listed

- Cost less then any of the other 3 mythic by 200g[/code]
- It gives 8% physicalwamp sustain that allows you to pick the Ultimate Hunter rune which is HUGE.(around 20% cd reduction on your R), also Legend: Alacrity which does not become an option, but more of a have to, since Xayah need a moderate amount of AS
- It deals 6% max hp physical damage if you hit someone with 2 separate attack, gives you 15% movement speed, and a little shield, that scales with ad for 2 second, all of this on a 16s cd
- Mythic bonus. giving 4% armor penetration per legendery item

Conclusion:
Better tank shredder then Kraken and just more damage in general to give to it's 1 item powerspike.
sustain, wich does not better than shieldbow's, but you can benefit from it from your spells too, so you don't have to auto attack for it.
It does not give you consant but for a duration better movementspeed then Galeforce, better active item damage, because it deals physical damage, benefits from physical wamp, and scales with the enemies hp and on a shorter cooldown.

With 3 items (Eclipse, Lord, Collector), you're gonna have 40% armor penetration 30 lethality and 40% crit,
you will melt through every tank, and chunk of half of a mage's hp with just 3 feathers, not to mention that the 5% execute also becomes more vialbe the tankier the enemy is,
You should always build Lord Dominik second though better than collector in almost all cenarios

The reason you buy Navori before IE is simple, It's better at that point, gives you same amount of damage, but it gives a huge amount of ability haste, plus 20% off from all the non-ultiamte cooldown after a crit, so you can spam your already high damage abilities faster, IE only increases your AA damage which is something that we already know we don't want to rely on. after Navori you can buy IE, or rfc if the enemy don't have a single tank nor bruiser, and you need to get the first shot in against high ranged characters.

30%tenacity + 300hp is good against most comps, you are going to have enough dmg in the mid/late game anyway, but if there's absolute no threat to you then go for wrath obviously.

I personally don't like it, but if you can't live without it, buy it instead of collector.

Death dance: gives great damage/armor ratio
Dead Man's plate: Bonus movement speed always good on Xayah, the Statik damage scales with ad
Randuin: if you REALLY need armor

NO
you need the scaling attackspeed from lethal tempo, you damage is still not built around bursting.